1. Field of the Invention
The present invention relates to a method for forming a semiconductor-thin-film, particularly to a method for forming a polycrystalline silicon (poly-Si) film for a thin-film transistor (TFT) used for a liquid-crystal display or an IC.
2. Description of the Prior Art
It has been recently possible to provide a liquid-crystal display having a driving circuit using a poly-Si TFT formed on an inexpensive glass substrate. In view of low process temperature and a high throughput, an excimer laser crystallization method is known for forming such a poly-Si thin film, wherein an amorphous-silicon (a-Si) thin film formed on the glass substrate is crystallized by irradiating an excimer laser beam thereon.
The excimer laser crystallizing method has a problem that uniformity in electrical characteristics cannot be obtained. To avoid the problem, a so-called cap annealing method is disclosed in the Japanese Patent registered No. 2776276 issued on May 1, 1998, wherein an SiO2 thin film is deposited on an a-Si thin film as a cap film prior to performing laser crystallization process for flattening a surface of a poly-Si thin film.
However, the cap annealing method has another problem that a crystal-grain size of the poly-Si film is limited because the interface between the cap film and the a-Si thin film serves as a nucleation site for crystallization. Therefore, a field-effect mobility of a poly-Si TFT remains at approx. 100 cm2/Vs. Although a liquid-crystal display can be realized by using such low mobility, it is impossible to realize a high-integration circuit such as a DRAM to be driven at a high frequency.
Moreover, as disclosed in the Japanese Patent Laid-Open Nos. 267988/1994 and 69967/1996, it is also known to utilize a catalyst element such as nickel for accelerating crystallization of an a-Si thin film.
However, when utilizing a catalyst element, it remains in a poly-Si thin film. Therefore, a special process such as heat treatment is necessary for removing the catalyst element as disclosed in the Japanese Patent Laid-Open No. 233363/1998.
It is, therefor, an object of the present invention to solve the above problems and provide a new method for forming a poly-Si thin film having a large crystal grain size and a uniform electrical characteristics without increasing the number of steps.
A semiconductor-thin-film forming method according to the present invention enlarges a grain size of an obtained poly-Si thin film by adding a catalyst element for accelerating crystallization of an a-Si (or recrystallization of a poly-Si having a small crystal grain size) into a cap film in a cap annealing method for obtaining the poly-Si thin film by using an excimer laser crystallizing method and thereby irradiating a laser beam on the a-Si thin film through the cap film which is made of a transparent insulating film.
By using the cap film, the catalyst element does not deeply doped into the a-Si film. The catalyst element shallowly doped into the a-Si film precipitates on the interface between the poly-Si film and the cap film after laser annealing. By removing the cap film, the precipitated catalyst element is also removed together with the cap film. Accordingly, a new catalyst-element-removing step is unnecessary and thus, the amount of the catalyst element in the poly-Si film is greatly reduced.
To achieve the above object, the present invention basically has the following aspects.
That is, the first aspect of a semiconductor-thin-film forming method of the present invention is characterized by successively forming a non-single-crystal thin-film semiconductor, i.e., amorphous semiconductor film or polycrystalline semiconductor film having a fine crystal grain size, and a cap film containing a catalyst element in order on an insulating substrate. Then a high-output laser beam such as an excimer laser beam is irradiated on the amorphous film through the cap film to transform the amorphous film into polycrystalline film. After that, the cap film is removed.
The second aspect of the present invention is characterized by doping a catalyst element into the cap film for accelerating crystallization of a non-single-crystal thin-film semiconductor film prior to irradiating the excimer layer beam on the amorphous film through the cap film.
The third aspect of the present invention is characterized by forming the cap film by using a sputtering method or a hot-wire CVD method.
The fourth aspect of the present invention is characterized by using the cap film having a thickness of at least 10 nm.
The fifth aspect of the present invention is characterized by removing the cap film by using a dry etching method using a gas containing halogen.
The sixth aspect of the present invention is characterized by including an RCA cleaning method in removing process of the cap film.
The seventh aspect of the present invention is that the above mentioned catalyst element is selected from a group consisting of Ni, Pt, Pd, Co, Fe, Cu, Au, Ag, Cr, and mixture thereof as the catalyst element or elements.
The eighth aspect of the present invention is characterized by using a silicon oxide film as the cap film.